The regulation of tyrosine phosphorylation is critical in the growth and guidance of axons during the development of the vertebrate nervous system. Accumulating evidence suggests that receptor-type tyrosine phosphatases (RPTPs) play key roles in the signaling processes underlying axon growth. Several major classes of RPTP possess extracellular domains (ECDs) that are structurally similar to those of cell adhesion molecules (CAMs). Our overall hypothesis is that these CAM-like RPTPs, which are expressed on the surfaces of developing neurons, are involved both as regulatory ligands and as neuronal receptors in the regulation of vertebrate axon growth. Work in the previous granting period has established that 2 vertebrate RPTPs, PTP-delta and PTPRO, are strong candidates for axon growth regulation. In vitro, the ECD of PTP-delta is a neurite-promoting neuronal cell adhesion molecule that mediates attractive growth cone steering. PTP-delta can bind homophilically, and may be both a regulatory ligand and a neuronal receptor for these attractive cues. In contrast, the ECD of PTPRO is a neurite-inhibitory repulsive guidance cue in vitro. Knockdown of PTPRO or of PTP-delta expression in embryonic motor neurons leads to changes in axon growth. This proposal is designed to build on these observations. In Aim 1, site-directed mutagenesis will be used to determine the regions of the PTP-delta and PTPRO ECDs responsible for functional binding. In Aim 2, chimeric receptor production, neuronal transfection, and growth cone migration assays will be used to identify and characterize the receptor functions of PTP-delta. In Aim 3, biochemical and functional approaches will be used to characterize PTPRO/Trk interactions and characterize novel PTPRO substrates. Finally, Aim 4 will use both the mouse and chick systems to characterize the in vivo roles of PTP-delta, PTPRO, and other CAM-like RPTPs in specific vertebrate axon guidance decisions. Our experiments will move the field closer to a molecular understanding of the role of tyrosine phosphorylation in vertebrate axon growth, by elucidating the roles of RPTPs. This is a critical background for efforts to understand how these processes go awry in developmental disorders of the brain and spinal cord.